I always steer clear of attacks on Michael Martin, the Speaker. They seem to have started with a feud between him and various parliamentary sketchwriters, who were both anti-Labour and anti-Scottish. They called him Gorbals Mick, which, as he has never lived in that notorious area of Glasgow, tells you more about them than him. He fell out with a couple of key members of staff, but it's always hard to know the truth about office politics. And though he might not have been the most decisive figure in the chair, he didn't seem particularly biased. His genial good-humour has often calmed the chamber at awkward moments. Not a genius, but not a disaster either.

Anyway, the Speaker of the Commons is one of those rare authority figures it is in almost everyone's interests to protect and support. He or she comes under intense pressure from the government and is the symbol of parliamentary power, the bedrock of the system and all our liberties (well, those we have been kindly allowed to keep). It seems almost indecent to attack the Speaker, in a way it doesn't for ordinary MPs. Yet, reluctantly, I have to say that he should go.

The attacks on Martin are not about class, as his allies claim. He was, it's true, a sheet metal worker and a factory shop steward, a Labour MP of the old sort, whose Scottish accent has been much mocked by snobs. But he is certainly not the first humbly born figure to become Speaker. His predecessor, Betty Boothroyd, former Tiller Girl and daughter of Yorkshire textile workers, was hardly born with a silver spoon in her mouth. Bernard Weatherill, perhaps the best known Speaker of recent times, began his career crosslegged in his father's sporting tailor's business, and overheard one Tory MP complain, when he was first elected, that he didn't know what the Commons was coming to: "They've even let my tailor in now."

Nor has Martin been by any means "the worst-ever Speaker", as is now alleged. As I say, he's had good points in the chair, as well as sometimes being a bit slow to haul ministers up (and to stop barracking backbenchers). In the history of the job there have been serious drunks and serial incompetents far worse than Speaker Martin. In some ways, he was a breath of fresh air. He hosted charity receptions with a genuine warmth - he really does have a good heart. Those who have raised money for good causes with his help say he has been remarkably unstuffy and kind in private, not standing on his dignity or being grand. He has always been touchingly proud and admiring of his wife.

Yet he has been doing the job at a time when, with hindsight, it should have been done by a Tory or Liberal Democrat MP, preferably from England. It was a brilliant Labour operation to get him the job, which now looks not very brilliant at all. The Scots-dominated Labour cabinet has been in power for so long that many backbenchers, never mind voters, deeply resent its hegemony. Tony Blair's contempt for the Commons ought to have been answered by a vigorous, chippy parliamentarian in the Speaker's chair, who wasn't overflowing with goodwill. The arrival of Gordon Brown, part of the same small world of Scottish Labour as Martin, has made his position even harder.

All of that is good reason for him to decide that, after nearly eight years in the job, it is time to step down. But there is now a better reason still. Somebody in his office has lied about taxi bills run up by his wife. There have also been questions about the use of public funds to support his PR operation; about air miles earned on public duties being used by his family; and now about £75,000 claimed for expenses on a constituency home with no mortgage. The taxi row has caused the resignation of the former civil servant and PR man Mike Granatt, who is very well known at Westminster and liked and trusted by every journalist I know who's dealt with him.

None of this is surprising or heinous. Plenty of MPs go right to the limit in claiming what they can for housing and other costs - MPs who would never dream of going further and paying family members who didn't do much work, or of being overtly dishonest. Martin, it seems, has behaved in line with an expenses culture that grew up in parliament in the early years of the 20th century and has only been seriously challenged in recent years. So they've all been at it - or most of them. So what?

The answer to that lies in the collapse of public trust in politicians. The polls have been telling us the same message for years. But the numbers are truly shocking. One poll last month showed that 83% of those asked said they didn't trust politicians, and that just 4% believed MPs put the country's interests first. This is dire. This is the beginning of the end of decent parliamentary politics. And you simply can't deal with it when the Speaker, the man ultimately in charge, has the slightest cloud hanging over him.

When I first worked in the Commons, it seemed almost accepted that MPs jacked up their expenses. It wasn't regarded as wrong. It was just the same in journalism. Nebulous "contacts" were cited for pricey lunches, drinking sessions and so on. Hacks swapped bundles of restaurant and taxi receipts and scribbled bills on each other's "blankies". Papers and television companies had informal tariffs about the levels of expense claims expected for journalists at different levels of seniority. It was frankly corrupt. It was dishonest and demeaning. Now that's all gone (as far as I can tell), swept away by harder times and collective embarrassment.

A similar cleaning-out has to happen in parliament. MPs hate and resent serious oversight by an outsider, the parliamentary commissioner. They stand on their dignity and independence, even when the public are laughing at them. MPs need utterly transparent rules and a new body to oversee and monitor all employees. They need a fresh start.

That can only come with a new Speaker, freely elected by MPs after he or she has promised to root out the culture of petty fraud in Westminster and bring the place into line with the cleanest employment practices in the country. I don't think Michael Martin should resign this week, or next: you can't begin a campaign to restore the dignity of parliament by letting press attacks hound the Speaker out. He should be allowed to make his decision and announce it in his own time. But it's now clear what that decision should be. When a Speaker is part of the problem, he owes it to parliament to step aside.